Digital camera based vision systems have been installed in automotive passenger compartments for various safety-related purposes such as identifying the presence and position of passengers, and determining the driver's state of awareness. Such systems include active light sources to ensure that the objects of interest are adequately illuminated, and the light impinging on the camera chip includes reflected components of both active and ambient light sources. While ambient lighting (solar and otherwise) increases illumination of objects in the passenger compartment, it is subject to drastic and unpredictable variation, and usually produces undesired shadowing due to non-uniform illumination of any given object of interest. Clearly, vision system performance in the automotive environment could be improved by somehow minimizing or eliminating the effects of ambient lighting.
Various techniques have been devised for minimizing the effects of ambient lighting in active illumination vision systems, but not without significant cost and/or performance penalties. For example, the active light source can be restricted in wavelength to a range in which sunlight is strongly absorbed by the atmosphere, making the vision system essentially insensitive to solar ambient illumination. However, silicon-based camera chips are relatively insensitive to light in that wavelength range, and significant cost penalties are incurred in compensating for the reduced sensitivity. Another technique involves pulsing the active light source so that alternate video frames are influenced only by ambient lighting; the effects of ambient lighting are eliminated by subtracting the ambient-only data from the combined illumination data. However, this technique incurs significant performance penalties by halving the frame rate and by introducing blurring artifacts if the objects of interest are in motion. What is desired is way of minimizing the effects of ambient lighting without incurring such penalties.